1. Field of the Invention
This invention concerns a laser processing apparatus for cutting plate-like workpieces by laser beams and, in particular, it relates to an improvement in the processing accuracy by the accurate detection of the position of the optical axis.
2. Description of the Prior Art
In a laser processing apparatus, processing is generally carried out under the control of a numerical control device or a computer. In such laser processing apparatus, it is required to exactly set the positional relation between an original position or reference position for initially positioning a plate-like material to be processed and a processing position where the optical axis of laser beams is to be located. The original position is indicated with reference to the optical axis of the laser beams. Accordingly, upon setting a positional input to a control device, such as a numerical control device, the optical axis of the laser beams is positioned from the original position to the position for which the input is set. Then, the subsequent positioning for the workpieces is effected based on the original position as the reference in accordance with the signals for the coordinate values inputted to the control device. When laser processing, such as weld-cutting, is carried out after the positioning of the workpiece, there are no problems if the positional relation between the original position and the optical axis of the laser beams is always maintained exactly. In the laser processing apparatus, however, slight deviation may sometimes be caused to the axial center position of the laser beams due to the exchange of a condenser lens and the like or due to an error in the optical path system during use. This has an undesired effect on the fabrication accuracy. Since a carbon dioxide gas laser in the form of invisible rays is generally used, it has been rather difficult to detect the deviation of the optical axis of the laser beams.